arXiv subjects

M. Hernández Villanueva

Publications and source records attributed to M. Hernández Villanueva.

6 recordsLinked to original sources

arXiv2024

Search for a dark leptophilic scalar produced in association with $τ^+τ^-$ pair in $e^+e^-$ annihilation at center-of-mass energies near 10.58 GeV

A dark leptophilic scalar $(ϕ_L)$ is a hypothetical particle that couples only to leptons rather than quarks. We report on a search for $ϕ_L$ in the $e^+e^- \to τ^+ τ^- ϕ_L, ~ϕ_L \to \ell^+ \ell^- ~(\ell = e, μ)$ process using 626~\invfb of data collected by the Belle experiment near the \Y4S resonance. We validate the backgrounds with multiple control regions in data, using a novel multiclass multivariate event classifier. In absence of a signal, we quote upper limits at the 90\% confidence level on the coupling between $ϕ_L$ and leptons. Our bounds, obtained in a blinded approach, are 19\% more constraining than the previous limits, averaged over the mass range $0.04 \leq m_{ϕ_L} \leq 6.5~\gev$. We exclude the parameter space below 4~\gev favored by measurement of the anomalous magnetic moment of the muon.

hep-ex↗

arXiv2023

Search for a heavy neutrino in $τ$ decays at Belle

We report on a search for a heavy neutrino in the decays $τ^- \to π^- ν_h$, $ν_h \to π^\pm \ell-+$, $\ell = e, μ$. The results are obtained using the full data sample collected with the Belle detector at the KEKB asymmetric energy $e^+e^-$ collider. We observe no significant signal and set 90% CL upper limits on the couplings of the heavy right-handed neutrinos to the conventional SM left-handed neutrinos in the mass range 0.2-1.6 GeV/c$^2$.

hep-ex↗

arXiv2023

Measurement of Angular Coefficients of $\bar{B} \to D^* \ell \barν_\ell$: Implications for $|V_{cb}|$ and Tests of Lepton Flavor Universality

We measure the complete set of angular coefficients $J_i$ for exclusive $\bar{B} \to D^* \ell \barν_\ell$ decays ($\ell = e, μ$). Our analysis uses the full $711\,\mathrm{fb}^{-1}$ Belle data set with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the $B \to D^*$ transition and the Cabibbo-Kobayashi-Maskawa matrix element $|V_{\rm cb}|$. Using recent lattice QCD calculations for the hadronic form factors, we find $|V_{\rm cb}| = (41.0 \pm 0.7) \times 10^3 $ using the BGL parameterization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter $w$, and investigate the differences of the electron and muon angular distributions. We find no deviation from Standard Model expectations.

hep-ex↗

arXiv2023

Search for the decay $B^0_s \rightarrow π^0 π^0$ at Belle

We report the results of the first search for the decay $B_s^0\rightarrowπ^0π^0$ using $121.4\ \rm fb^{-1}$ of data collected at the $Υ(5\rm S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We observe no signal and set a 90\% confidence level upper limit of $7.7\times 10^{-6}$ on the $B_s^0\rightarrowπ^0π^0$ decay branching fraction.

hep-ex↗

arXiv2023

Measurement of Differential Distributions of $B \to D^* \ell \bar ν_\ell$ and Implications on $|V_{cb}|$

We present a measurement of the differential shapes of exclusive $B\to D^* \ell \barν_\ell$ ($B = B^-, \bar{B}^0 $ and $\ell = e, μ$) decays with hadronic tag-side reconstruction for the full Belle data set of $711\,\mathrm{fb}^{-1}$ integrated luminosity. We extract the Caprini-Lellouch-Neubert (CLN) and Boyd-Grinstein-Lebed (BGL) form factor parameters and use an external input for the absolute branching fractions to determine the Cabibbo-Kobayashi-Maskawa matrix element and find $|V_{cb}|_\mathrm{CLN} = (40.1\pm0.9)\times 10^{-3}$ and $|V_{cb}|_\mathrm{BGL} = (40.6\pm 0.9)\times 10^{-3}$ with the zero-recoil lattice QCD point $\mathcal{F}(1) = 0.906 \pm 0.013$. We also perform a study of the impact of preliminary beyond zero-recoil lattice QCD calculations on the $|V_{cb}|$ determinations. Additionally, we present the lepton flavor universality ratio $R_{eμ} = \mathcal{B}(B \to D^* e \barν_e) / \mathcal{B}(B \to D^* μ\barν_μ) = 0.990 \pm 0.021 \pm 0.023$, the electron and muon forward-backward asymmetry and their difference $ΔA_{FB}=0.022\pm0.026\pm 0.007$, and the electron and muon $D^*$ longitudinal polarization fraction and their difference $ΔF_L^{D^*} = 0.034 \pm 0.024 \pm 0.007$. The uncertainties quoted correspond to the statistical and systematic uncertainties, respectively.

hep-ex↗

arXiv2017

Effective-field theory analysis of the $τ^- \to η^{(\prime)} π^- ν_τ$ decays

The rare $τ^- \to η^{(\prime)} π^- ν_τ$ decays, which are suppressed by $G$-parity in the Standard Model (SM), can be sensitive to the effects of new interactions. We study the sensitivity of different observables of these decays in the framework of an effective field theory that includes the most general interactions between SM fields up to dimension six, assuming massless neutrinos. Owing to the strong suppression of the SM isospin breaking amplitudes, we find that the different observables would allow to set constraints on scalar interactions that are stronger than those coming from other low-energy observables.

hep-ph↗